ABSTRACT Macrophages infected with Mycobacterium tuberculosis (Mtb) characteristically accumulate lipid droplets and arrange into specialized structures called granulomas. Cholesterol is required for persistent Mtb growth, but the specific mechanisms by which cholesterol is sequestered and utilized remain an area of active investigation. Nuclear hormone receptor modulation has been implicated in controlling the influx of cholesterol into infected macrophages. However, I recently discovered that mycobacterial infection may also regulate macrophage cholesterol levels via modulation of cholesterol efflux. I hypothesize that pathogenic mycobacteria are able to inhibit cholesterol efflux from infected macrophages in order to cell autonomously increase and maintain intracellular cholesterol content. This regulation may provide a source of nutrition for sustained bacterial growth and this accumulation comes in addition to increased cholesterol influx resulting from pathogen-directed immune modulation of host macrophages. I have shown an increase in both cholesterol and lipids specifically in infected macrophages not previously demonstrated and at early infection time points. I have shown that apolipoproteins, key cholesterol efflux molecules, are dramatically downregulated cell-autonomously in infected macrophages isolated from whole animals. Hence, in this proposal I aim to (1) investigate the role of pathogen and immune pressures on cholesterol enrichment and disease burden and (2) determine the role of infection-modulated apolipoproteins in control of macrophage lipid composition. I will carry out aim (1) by examining the direct nutritional effect of host cholesterol utilization by comparing infection by mutant Mm strains defective in cholesterol utilization with wildtype infections, evaluating the macrophage polarization state of infected versus uninfected macrophages and using genetic and pharmacologic tools to alter cholesterol content and macrophage polarization state in order to determine the influence of cholesterol on the immune properties of host macrophages and vice versa. I will carry out aim (2) by using apolipoprotein overexpressing lines to examine the effect of apolipoprotein recovery on infection and cholesterol accumulation, investigating the role of nuclear receptor LXR? in regulation of apolipoprotein expression, and by directly assessing apolipoprotein expression through development of transgenic reporter lines allowing for direct visualization of expression dynamics during infection. These works will contribute to identification of the mechanisms through which mycobacteria modulate intracellular cholesterol within macrophages, defining contributions to infection and the immune modulatory effects of cholesterol accumulation as well as defining a new mechanistic role for the apolipoproteins in mediating the accumulation of macrophage lipids and in progression of mycobacterial disease.